The present invention relates to a method for the manufacture of a microwave delay line. It also relates to microwave delay lines obtained by this method and to the travelling wave tubes having said lines.
As is known the delay line associated with a travelling wave tube functioning as an amplifier ensures the interaction between an electron beam focussed along the axis of the line and the fundamental propagation mode of the hyperfrequency travelling wave traversing the line, with a phase velocity close to that of the beam. The electrons transfer energy to the hyperfrequency wave and consequently there is an amplification phenomenon of the microwave energy circulating in the line.
Helical delay lines are conventionally used in travelling wave tubes. The disadvantage of these lines is that they excite oscillations in the travelling wave tubes for powers of the order of a few kilowatts.
Therefore delay lines having a more complicated construction are used and for which the amplitude of the inverse travelling mode of the wave, which causes the oscillation of the travelling wave tube, is reduced compared with that of a helical line. Thus, these lines have a reduced tendancy to bring about the excitation of oscillations of the travelling wave tubes and travelling wave tubes of a few dozen kilowatts have been obtained. Thus, it is possible to use double helix lines, constituted by two identical helixes of the same longitudinal axis coming from the same point and having opposite winding directions. Ring and bar or ring and loop lines are also widely used. They comprise rings arranged perpendicularly to the propagation direction of the electron beam, two successive rings being connected by a bar, giving the ring and bar line, or by a loop, giving the ring and loop line, and two successive bars or loops are diametrically opposed with respect to the rings.
The main disadvantage of double helix delay lines and of ring and bar and ring and loop lines is that they are difficult to construct. Thus, it is known to make double helix lines by welding together two identical helixes, which is very difficult or by milling or the electro-erosion of a cylindrical tube, which is also difficult to carry out. The ring and bar type lines can also be produced by machining a cylindrical tube. Ring and loop lines are generally obtained by using a mechanically or chemically cut metal strip, which is then bent back on itself in alternating manner and with the desired pitch. Problems are encountered if it is necessary to manufacture a line of the ring and loop type by this method using a metal such as tungsten which is difficult to treat mechanically when in the form of a strip, but which is in great demand, mainly due to its refractory characteristics.